The new generations of decoders, in particular those dedicated to high definition, have to handle new video compression standards (such as MPEG4), demodulate DVB-S2 type signals for satellite reception and offers new functionalities by virtue of the presence of a hard disk. These new items of equipment give rise to an increase in the processing power which has to be achieved inside the apparatus and therefore the consumption of energy. Hence, the electronics contained in this new generation of decoders consumes more energy than contemporary decoders. This energy gives rise to an increase in temperature which, eventually, is prejudicial to the reliability of the integrated circuits and of the other sensitive elements such as chip cards and hard disk. To remedy this problem, a cooling system designed with the aid of fans which blow fresh air from the outside into the whole of the cavity of the decoder and which extract the heat are implemented in decoders.
A fan is disposed in proximity to the vents so as to accelerate the exchanges with the outside and improve cooling. But the use of a fan at full speed is noisy. If the apparatus is a television decoder for example, it may be placed in a bedroom. In this case, it must not disturb the sleep of the users, the maximum threshold usually allowed is twenty-five decibels.
To reduce the sound level, constructors controls the fan at a certain moment and modulates its speed as a function of a preset speed to be attained. A temperature probe is placed in a usually hot location of the apparatus representative of the various hot spots of the apparatus. As soon as a threshold is attained or exceeded, the fan is turned on and its speed depends on the deviation between this threshold and the temperature measured by the probe, or else always turned on at a low speed so long as a temperature threshold is not exceeded. Typically, the fan is set to idle during power-up of the apparatus, and as soon as the temperature exceeds 50° C., the speed of the fan accelerates. An improvement consists in defining several temperature thresholds and as a function of these thresholds in dispatching a certain control value. To control its effectiveness, the fan has a tachometer probe which emits a signal dependent on the actual speed of the blades. The fan control unit analyses the signals originating from a probe and ensures regulation of the internal speed of the apparatus. The regulation is performed by an auto-control of PDI type (P for proportional, D for differential and I for Integral and). This efficacious auto-control allows accurate regulation of the speed, by adjusting the command provided to the fan as a function of the values measured by the tachometer probe so as to attain a preset speed.
Measurements have shown that this device turned out to be noisy, especially at power-up. At this moment, the fan goes from a zero speed to a preset speed. It is important that start-up be progressive, or at least that the speed on start-up remain less than the steady-state preset speed, and that start-up not cause any oscillation, so as not to be too noisy. Tests have shown that if a start-up command of significant amplitude is not applied, the fan does not start and/or merely oscillates. However, prolonged application of the start-up command causes the actual speed of the fan to overshoot and renders its operation more audible, this being reinforced by the fact that thereafter it is necessary to slow the fan down in order for it to attain its preset speed, the slowing down is also audible.
This document JP 2001 037274-MURATA published on 9 Feb. 2001 describes an electric motor evidently controlled by pulsations (see the diagram). During a first power-up, initialization parameters are applied and the speed of the motor is measured. If the speed of the motor is below a preset speed, then the value of the voltage is increased. By repeating this operation several times until the preset value is attained, the desired speed is obtained. The voltage parameter is then saved for all subsequent power-ups.
Therefore, there is a necessity to modify the control value of the fan to reduce the variations of speed, and consequently the fan start-up noise.